The present invention relates to a centrifugal multiblade blower suitable to an automotive air conditioning system.
In automotive air conditioning systems, there is usually employed a centrifugal multiblade blower fan installed upstream of an air duct. One such centrifugal multiblade blower has been disclosed in Japanese Patent Provisional Publication No. 64-41700 (corresponding to Japanese Patent No. 2690731). FIG. 7 is a cross section showing the structure of the centrifugal multiblade blower disclosed in the Japanese Patent No. 2690731. Centrifugal multiblade blower a shown in FIG. 7 is comprised of a multiblade fan b formed with a plurality of blades b1, a blower fan motor c, and a scroll casing d that accommodates therein the multiblade fan b and defines a scroll chamber d1 between the inner periphery of the casing and the outer periphery of the multiblade fan. Multiblade fan b is installed onto a motor shaft c1 of fan motor c. Casing d is formed into a logarithmic spiral shape and comprised of a suction-side case plate d3 formed with a suction port d2 and a fan-motor-side case plate d4 located opposite to the suction-side case plate d3. A motor body c2 of fan motor c is attached to the motor-side case plate d4. The radius R of the logarithmic spiral scroll casing is generally defined by an expression R=R0exp{n(xcex8+xcex80)}, where R0 denotes a radius of the multiblade fan, xcex8 denotes an angle measured in the direction of rotation of the multiblade fan from a central point of a tongue portion of scroll casing that defines the narrowest portion of the scroll chamber, xcex80 denotes an angle from a point across which a length L1 of the scroll chamber (often called a scroll width) measured in the axial direction of the motor shaft begins to enlarge to the central point of the scroll-casing tongue portion, and n denotes a so-called enlargement angle that represents the magnitude of enlargement of the scroll chamber in the radial direction of the multiblade fan (see FIG. 6). In centrifugal multiblade blower fans used for automotive air conditioning systems, the enlargement angle n is usually set to range from 5 degrees (8.72xc3x9710xe2x88x922 radians) to 8 degrees (14.0xc3x9710xe2x88x922 radians). As is generally known, the volumetric capacity of the scroll chamber tends to increase, as the enlargement angle n increases, and thus the scroll casing is enlarged in the radial direction of the multiblade fan. In other words, the volumetric capacity of the scroll chamber tends to decrease, as the enlargement angle n decreases, and thus the scroll casing is reduced. For the reasons set out above, with the enlargement angle n set to a comparatively smaller angle, it is possible to down-size the scroll casing, but the volumetric capacity of the scroll chamber tends to decrease undesirably. Owing to the decreased volumetric capacity of the scroll chamber, during operation of the centrifugal multiblade blower, there is an increased tendency for the counter-flow rate of air flowing from a suction-side aperture G1 defined between the multiblade fan b and the suction-side case plate d3 toward the suction port d2 to increase. At the same time, there is an increased tendency for the counter-flow rate of air flowing from a motor-side aperture G2 defined between the multiblade fan b and the motor-side case plate d4 toward the upstream side of the scroll chamber d1 to increase. Therefore, in the centrifugal multiblade blower a, although the scroll casing can be down-sized by reducing the enlargement angle n of the scroll chamber, the fan efficiency is reduced. Additionally, due to the reduced enlargement angle n, the pressure in the scroll chamber tends to become unstable. This may increase noises and vibrations during operation of the multiblade fan.
Accordingly, it is an object of the invention to provide a centrifugal multiblade blower, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a centrifugal multiblade blower, which is capable of down-sizing a scroll casing by reducing a so-called enlargement angle of a scroll chamber, without lowering a fan efficiency and without increasing noises and vibrations.
In order to accomplish the aforementioned and other objects of the present invention, a centrifugal multiblade blower comprises a multiblade fan having a plurality of blades, a fan motor having a motor shaft on which the multiblade fan is mounted, a scroll casing that accommodates therein the multiblade fan and has a discharge port and cooperates with an outer periphery of the multiblade fan to define a spiral scroll chamber, the casing comprising a suction-side case plate having a suction port, and a motor-side case plate which is located opposite to the suction-side case plate in such a manner as to sandwich the multiblade fan between the suction-side case plate and the motor-side case plate, and on which a motor body of the fan motor is mounted, a first counter-flow prevention means for preventing part of air flowing through the scroll chamber from flowing through a first aperture defined between the multiblade fan and the suction-side case plate back to the suction port, and a second counter-flow prevention means for preventing part of air flowing through the scroll chamber from flowing through a second aperture defined between the multiblade fan and the motor-side case plate back to an upstream side of the scroll chamber, wherein a length of the scroll chamber measured in an axial direction of the motor shaft is dimensioned to be longer than a length of the multiblade fan measured in the axial direction of the motor shaft, and the scroll chamber is gradually enlarged toward the discharge port of the casing. It is preferable that the scroll chamber is gradually enlarged in the axial direction of the motor shaft at an axial enlargement angle xcex1 representative of a magnitude of enlargement of the scroll chamber in the axial direction of the motor shaft toward the discharge port, and additionally the scroll chamber is gradually enlarged in a radial direction of the multiblade fan at a radial enlargement angle n representative of a magnitude of enlargement of the scroll chamber in the radial direction of the multiblade fan from a tongue portion of the scroll casing toward the discharge port. The radial enlargement angle n is defined by an expression R=R0exp{n(xcex8+xcex80)}, where R denotes a radius of the scroll casing, R0 denotes a radius of the multiblade fan, xcex8 denotes an angle measured in a direction of rotation of the multiblade fan from a central point of the tongue portion that defines the narrowest portion of the scroll chamber, and xcex80 denotes an angle from a point across which the length of the scroll chamber measured in the axial direction of the motor shaft begins to enlarge to the central point of the tongue portion.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.